Mobile electronic devices such as cellular phones are increasingly used all over the world, and their communication performance has been much improved. Electronic devices such as notebook-type personal computers (notebook PCs) with wireless communication capability such as wireless LAN or Bluetooth (registered trademark) are also increasingly used. For such wireless communication, there are standard specifications including IEEE802.11b, which achieves a transmission rate of 11 Mbps in the 2.4 to 2.5 GHz band, and IEEE802.11a, which utilizes 5 GHz band (5.15 to 5.825 GHz). An electronic device with wireless communication capability employs a wireless antenna for sending and receiving radio wave (electromagnetic wave) in these bands. An antenna used for such an electronic device may be a whip (rod) antenna, which has even sensitivity in any direction as a non-directional antenna, a dipole antenna, which consists of two antenna elements arranged rectilinearly as wire antennas (line-shape antenna), and an inverted F type antenna, which is a kind of planar antenna.
In a conventional technique disclosed in a publication, for obtaining a high gain of a inverted F type antenna in a desired direction, there is provided a structure that allows a metal member or dielectric member for frequency adjustment to be disposed between an antenna base plate and a plate-shaped F-type antenna radiation element in L-shape, and the position of the metal member or dielectric member is adjusted to obtain a high gain in a desired direction (for example, see Patent document 1). In another disclosed technique, when an antenna is mounted on a notebook PC, the effect of the dielectric constant of a display unit case on the antenna performance is adjusted by adjusting the gap between the antenna and the case, so that the resonance frequency of the antenna is matched with a target, central frequency.
In using a conventional wireless antenna, a frequency shift occurs when it is mounted on a notebook PC system, due to the tolerance of assembly or the tolerance of antenna itself. Such frequency shift occurs since there are a dielectric cover for a liquid crystal display (LCD) and a dielectric frame surrounding the LCD around the antenna element. The occurrence of the frequency shift strongly deviates the resonance frequency from the central frequency of 2.4 to 2.5 GHz, or 2.45 GHz, with a result that desired electromagnetic wave cannot be received and/or it cannot be radiated. This does not cause a serious problem, for example, when there is substantial space for an antenna to be placed that has a wide band covering the frequency shift, or when the antenna itself has a wide band. However, in the case of using an antenna having a narrow band, the frequency deviation significantly affects the performance of the antenna. The frequency deviation may be much worse than the specification of the antenna, resulting in the increased cost for the adjustment of frequency.
In a technique described in Patent Application No. 8-204438, the resonance frequency is changed by varying the capacitance between a radiation element and a metal shield case. Thus, in the technique described in Patent Application No. 8-204438, the capacitance changes depending on the adjustment of the frequency adjustment plate, which causes a problem of a reduced gain even if matching of resonance points is made. Therefore, the technique is not preferable when the antenna is used in a circumstance where it is sensitive to the gain.
In Patent Application No. 2003-37416, adjustment is performed for the effect on antenna, of the dielectric constant of the display unit case to which the antenna is fixed in contact therewith. With the technique of Patent Application No. 2003-37416, one type of antenna can be used for various devices, while error of mounting on a single type of devices cannot be handled. Specifically, devices of a single type have resonant points that are slightly different with each other due to the error in mounting. It is thus difficult, in the technique of Patent Application No. 2003-37416, to handle the deviation of the resonant points due to the error of mounting, the tolerance in size for each device type, or the like.
The present invention is made to solve the above technical problem, and has an object to make it possible to compensate a frequency shift occurring in an electronic device having an antenna.
Another object is to make it possible to perform the adjustment of frequency shift of an antenna after the final assembly of an electronic device.
Yet another object is, in an electronic device having an antenna, to allow the oscillation point to be shifted without substantially changing the capacitance and with avoiding an input on the gain.
The present invention addresses such a need.